In conventional metal surface-treating processes, a stationary, rinsing tank (usually called a recovery tank) is provided downstream of the surface-treatment tank to wash a treated article with water, or a one-staged or multi-staged running-water tank is provided to pre-treat an article to be surface-treated and the so pre-treated article is immersed in a surface-treating solution tank to effect the surface treatment on the article. The surface-treating solution used is continuously concentrated by a concentration device, and is then recycled to the surface-treating solution tank for reuse.
Acids such as mineral acids and organic acids and alkalis to be used as pre-treating solutions are examples of surface treating solutions; others are electroplating solutions, organic coating solutions, chemical treatment solutions, anodic oxidizing solutions and the like. Surface-treating solutions heretofore employed generally comprise from one to several inorganic chemicals acting as main ingredients, and small amounts of organic or inorganic auxiliary chemicals and additives incorporated as gloss-levelling agents, agents for removing impurities or draining agents. In the use of surface-treating solutions containing such ingredients, the surface treatment is conducted by selecting appropriately the concentration of each ingredient, the temperature, the pH, the electric circuit and other conditions. Control or maintenance of such surface-treating solution is troublesome and requires complicated steps. More specifically, various foreign substances such as those contained in drippings coming from the preceding step, those formed by falling and dissolution of an article to be treated in the surface-treating solution, those contained in a surface-treating solution of the preceding step carried forward as a result of a damage of a fixture for supporting the article to be treated, and dust from the ambient air are incorporated into the surface-treating solution, so that substances adversely influencing the surface treatment are accumulated in the surface-treating solution. Accordingly, the problem of aging of the surface-treating solution is unavoidable in the conventional techniques.
At the step subsequent to the surface-treatment tank, a plated article is washed in a one-staged or multi-staged water-washing tank. Carryover or dragout of the surface treating solution used at the preceding step into the water-washing tank cannot be avoided, because it is carried forward to the water-washing tank together with the plated article. In order to prevent loss of effective and valuable chemical ingredients of the surface-treating solution, it is desirable to recover these chemical ingredients from that water-washing liquid for reuse in the process.
Further, many of the chemical ingredients introduced into the rinsing liquid from the surface-treating solution are harmful to humans, and therefore, an expensive discharge-treatment apparatus must be provided if it is desired to discharge such rinsing liquid outside the system.
A mist is usually generated from the surface-treatment tank; this mist may contain essential and valuable chemical ingredients of the surface-treating solution. If they are not recovered from the mist, an economic loss ensues, and moreover, these ingredients frequently are harmful to the human body. Therefore, it is essential to treat such mist effectively.
In conducting such a surface-treatment process, a continuous concentration method using a device for concentration of surface-treating solutions is very effective; however, since a gas, or air containing the above-mentioned poisonous mist generated in the surface-treatment tank, is generally employed as the gas circulated in the concentration device, the evaporation capacity of the concentration device will vary depending on the change of the wet-bulb temperature, though the temperature of the process solution circulated in the concentration device is maintained at a constant level, with the result that the surface-treatment operation becomes unstable. Further, this method is unsatisfactory in that the treatment cost is high.
In the metal surface-treatment process, it is frequently an important requirement to control the temperature of the treating solution in the surface-treatment tank within a certain narrow range. In the treatment method of the system where a concentration device is connected to the surface-treatment tank, the temperature of the solution in the surface-treatment tank is greatly influenced by the liquid temperature in the concentration device, which is another problem involved in the conventional techniques.
In the water-washing tank subsequent to the surface-treatment tank, there is a problem in that the number of water-washing tanks must be high in order to provide effective rinsing as well as effective recovery of the treatment-tank ingredients and hence, the apparatus space must be large.
Many attempts have heretofore been made to solve these problems involved in the conventional techniques, but none of them have succeeded in giving satisfactory results. Accordingly, it is now generally conceded in the art that it is impossible to solve all of the foregoing problems by a simple unit-treatment system, and the various problems are considered separately and it is now tried to solve these problems separately.